A rivet is a joining device intended to join together two or more workpieces each drilled with a hole. The rivet includes a shank which is intended to be inserted into the holes in the workpieces, after these holes have been brought into axial coincidence. The shank must be inserted into the holes in such a manner that part of the shank projects from each of the ends of the passage formed by the juxtaposed holes. The shank is deformable and/or is combined with a deformable ring which forms part of the rivet.
In order to join the workpieces together, forces are applied to the rivet such that it deforms until it has, in the vicinity of the two ends of the passage, enlarged parts in clamping contact with the corresponding workpieces.
The deformation of the rivet may be achieved using slowly-acting forces or by single or repeated shocks.
It is often important, during deformation of the rivet, for the movements of the workpieces to be joined, or the stresses which they experience, to be small and/or tightly controlled.
Deformation induced by slowly-acting forces allows the movements of the workpieces and their stresses to be accurately controlled, but it requires heavy tooling in order to apply large forces.
Shock-induced deformation requires much lighter tooling, but it is difficult to control the position of the workpieces to be joined and may subject them to high stresses.
In the case of shock-induced deformation, an "anvil" is normally used, that is to say a piece which may be considered as being fixed and non-deformable, one end of the rivet is placed so as to bear on the anvil and the shock or shocks are exerted using a "riveting set" which acts on the opposite end of the rivet shank. This manner of operating is not entirely satisfactory from a theoretical standpoint since the deformation of that part of the rivet which is close to the anvil results in a slight movement, or deformation, of the workpieces. Furthermore, the need to have a fixed anvil or a large mass is an irksome constraint.
It may be imagined that it is more advantageous to exert the shocks on both ends of the rivet shank, but the manner in which the shock energy is applied to each of the ends of the rivet must be controlled very accurately in order to avoid movements of the rivets in its hole and of the workpieces to be joined or stresses on these workpieces.
More precisely, if the rivet to be formed initially consists of a homogeneous symmetrical cylindrical piece placed symmetrically with respect to the workpieces to be riveted, it is clear that the result, with regard to the movements and stresses imposed on these workpieces, will be all the better the smaller the difference between the kinetic energies of the two percussion tools and the shorter the time interval separating their impact on one end of the rivet. This will not be exactly the same in the case where the rivet to be formed is not symmetrical, and has a head, for example. Many other factors may be involved: for example, assuming that a shock is the very rapid application of a force on an object, the way in which this force varies is not without importance.
In order to simplify matters, the rest of the text will speak of "synchronous percussions" and of equal kinetic energies, it being necessary, however, always to take account of the reservations which have just been mentioned.
The document U.S. Pat. No. 3,704,506 proposes to execute "synchronous percussions" by providing, on each side of the rivet, a riveting set combined with a propulsion means which includes an electrical coil into which an electric current may be sent coming from the discharge of capacitors. The riveting set is firstly made to bear on the rivet and then the electric current sent into the coil gives the riveting set a force sufficient to deform the rivet.
The document U.S. Pat. No. 4,862,043 contains a critique of this prior-art process. According to this document, even if the riveting set is already in contact with the rivet before the operation, the prior art is of the "ballistic" type, that is to say the energy is supplied to the riveting set in a time appreciably shorter than that during which the material of the rivet, and of the workpieces, deforms, which would be the cause of deformations. U.S. Pat. No. 4,862,043 proposes to remedy this drawback by having a conformation which ensures that the force acting on the riveting set acts for a time which is approximately equal to that for deformation of the rivet and of the workpieces.
The electromagnetically actuated devices described hereinabove may be reproached on the grounds of being expensive, heavy and bulky.
Moreover, pneumatic riveting guns are known, see for example the document U.S. Pat. No. 4,039,034, with a piston which can move in a cylinder and a compressed-air accumulator intended to move the piston until it strikes a riveting set. These guns are provided with a manually actuated trigger. It is doubtful whether it is possible to combine them with means allowing, with sufficient accuracy, simultaneous triggering of two guns, equal and stable pressures in both accumulators and identical strokes for the pistons, in particular because of the difficulty of controlling the pressure oscillations in the pipes and in the accumulator.
The document U.S. Pat. No. 3,562,893 provides a system having two riveting sets acting in opposition, one being actuated by compressed air and the other by an explosive charge triggered by the impact of the first riveting set on the rivet. There is no overall symmetry between the two tools, which do not operate in a really synchronous fashion. Tailoring this system to different types of rivets is evidently difficult.